It is often desirable to coat an article, substrate or film in order to modify the properties. A particularly desirable coating is that of a heat-sealable film, i.e., a film which is capable of being bonded to itself, another film or another substrate with the application of heat and/or pressure. In this manner, the article, substrate or film can be sealed to form structures such as bags or other packaging materials.
Laminates and single or multi-layer films are two packaging materials that often employ heat-sealable layers. Laminates are conveniently made by coating a substrate, for example, paper or film, with a heat-sealable layer by extrusion coating. Extrusion coating is a process whereby a polymer or blend of polymers is fed into an extruder hopper. In the hopper the polymer or blend is melted and passed through a die to form a web. The web is then extruded onto the substrate through a nip roll/chill roll interface, for example, so that the molten web is pressed onto the substrate. The substrate is cooled by the chill roll and the wound up at a winder.
Similarly, many different processes are often employed to make single or multi-layer films which are useful as packaging materials. Such processes can include bubble extrusion and biaxial orientation processes, as well as, tenter frame techniques. In order to facilitate sealing, the heat-sealable film is usually employed singly or as the outermost or innermost layer in the case of multi-layer films.
Laminates and single or multi-layer films having a heat-sealable film layer are often used in "form, fill, and seal" machines. These machines create from film a continuous stream of packages that are capable of being closed by film-to-film seals. Often such packages are sealed via heat seal jaws that apply heat and pressure to form the film-to-film seal closure.
The heat seal closures made via the heat seal jaws will often be strongest after the seal has cooled to ambient temperature. However, in order to increase the production capacity, the packages are often filled with product before the bottom seal has had time to completely cool, therefore, the polymers at the seal interface haven't completely solidified (or recrystallized) or they are still in a softened state. Thus, the closure must exhibit a sufficient strength very rapidly without the need for cooling to ambient temperature. Otherwise, the closure will be destroyed by the weight of the product when the package is filled.
"Seal strength" is the strength of a heat seal at ambient temperature after the seal has been formed and reached its full strength. However, as described above the properties of the seal at temperatures subsequent to formation but prior to cooling to ambient conditions are often important. The properties of seal strength at temperatures above ambient are often referred to as "hot tack" properties.
There are a number of different hot tack properties that are important for heat-sealable films. One important hot tack property is the "initiation temperature." The initiation temperature is the first temperature above ambient at which a seal can be formed by applying a given pressure to a given thickness of film for a given length of time. In general, lower initiation temperatures are desirable because less energy is required to be used to form the seal and also the less time it takes for the initial seal to form at a given seal jaw temperature. Thus, production rates are capable of being increased.
Another important hot tack property is "ultimate hot tack." Ultimate hot tack is the largest strength the seal has at temperatures above the initiation temperature. Usually it is desirable that the ultimate hot tack occurs at the lowest possible temperature. Another hot tack property that is generally desirable is a wide processing window such that the film exhibits a suitable seal strength as measured over a broad temperature range. Also generally desirable is high temperature hot tack such that the seal strength remains sufficient even at elevated temperatures.
The hot tack properties are often determined by the composition employed to form the film seal. In the past, compositions such as those described in U.S. Pat. No. 4,339,507 and U.S. Pat. No. 5,741,861 that are a mixture of, for example, low density polyethylene and linear low density ethylene hydrocarbon copolymer have been employed. Unfortunately, however, such compositions often have hot tack properties that may limit the production capacity. Thus, it would be desirable to discover new compositions that have improved hot tack properties.
Advantageously, a new composition has been discovered which gives superior hot tack properties. The composition comprises
(a) from about 2 to about 13 percent by weight of polypropylene which is a homopolymer or a copolymer derived from at least about 80 percent propylene monomer and less than about 20 percent alpha-olefin monomer based on the total weight of the monomers, and which has a melt flow rate of from about 1.0 to about 50 dg/minute as measured in accordance with ASTM D-1238, Condition 230.degree. C./2.16kg.
(b) from about 87 to about 98 percent by weight of a polymer selected from the group consisting of linear ethylene polymer, substantially linear ethylene polymer, low density ethylene polymer, and mixtures thereof;
wherein the linear or substantially linear ethylene polymer is characterized by: PA1 (1) a density of from about 0.87 to about 0.960 g/cm.sup.3, PA1 (2) a molecular weight distribution, M.sub.w /M.sub.n of less than or equal to about 5, and PA1 (3) a melt index, I.sub.2, as measured in accordance with ASTM D-1238, Condition 190.degree. C./2.16 kg of from about 0.5 to about 20.0 dg/minute; and PA1 wherein the low density ethylene polymer is characterized by: PA1 (1) a density of from about 0.91 to about 0.96 g/cm.sup.3, and PA1 (2) a melt index, I.sub.2, as measured in accordance with ASTM D-1238, Condition 190.degree. C./2.16 kg of from about 0.1 to about 20.0 dg/minute.
The composition is suitable for forming a film having improved hot tack and for use in coating substrates. In this manner, a heat-sealable film may be made for packaging materials. The packaging materials are beneficial in that they exhibit surprising and unexpected hot tack properties and facilitate production.
When used in extrusion coating, the compositions of the instant invention often form homogeneous extrudates, give stable extrusion rates at high take-off speeds, have acceptable neck-in, give good product properties such as hot tack, tensile strength, wide temperature range processability, and good tear and abrasion resistance.